Multiple Input Multiple Output (MIMO) has recently attracted much attention as a broadband wireless mobile communication technology. A MIMO system seeks to increase data communication efficiency by use of a plurality of antennas. Depending on whether the same data or different data are transmitted through antennas, MIMO techniques are classified into spatial multiplexing and spatial diversity.
Spatial multiplexing is characterized in that different data are transmitted through a plurality of Transmission (Tx) antennas simultaneously. Therefore, data can be transmitted at a high rate without increasing a system bandwidth. In Spatial diversity, the same data is transmitted through a plurality of Tx antennas, thus achieving transmit diversity. Space time channel coding is a kind of spatial diversity scheme.
Depending on whether a receiver feeds back channel information to a transmitter, MIMO techniques are also categorized into open-loop MIMO and closed-loop MIMO. Open-loop MIMO schemes include Bell Labs Layered Space-Time (BLAST) and Space-Time Trellis Coding (STTC). According to BLAST, the transmitter transmits information in parallel and the receiver detects signals by repeating Zero Forcing (ZF) or Minimum Mean Square Error (MMSE) detection. Thus as much information as the number of Tx antennas can be transmitted. STTC achieves transmit diversity and coding gain by utilizing space. Transmit Antenna Array (TxAA) is a closed-loop MIMO technique.
CoMP was proposed to improve the throughput of a user at a cell edge by applying advanced MIMO under a multi-cell environment. The use of CoMP may reduce Inter-Cell Interference (ICI) in the multi-cell environment. Multi-cell BSs may provide joint data support to a UE by a CoMP operation. Also, each BS may improve system performance by simultaneously supporting one or more UEs MS 1 to MS K. A BS may implement Space Division Multiple Access (SDMA) based on channel state information between the BS and UEs.
There are largely two CoMP operation modes, joint processing being a cooperative MIMO based on data sharing and Coordinated Scheduling/Beamforming (CS/CB) mode.
In a CoMP wireless communication system, a serving cell and one or more cooperating cells may be connected to a scheduler over a backbone network. The scheduler receives feedback channel information representing a channel state between a UE and each cooperating cell, as measured by the cooperating cell. For example, the scheduler may schedule cooperative MIMO information for the serving cell and one or more cooperating cells. That is, the scheduler may issue a command related to a cooperative MIMO operation directly to each cell.
One of major problems with the CoMP technology is that resource utilization is restricted in multiple cells. For example, if a cell uses a specific resource unit for a UE, two or more cells use the same resources to support the UE in a CoMP operation. Therefore, the whole throughput of multiple cells may be adversely affected.
Accordingly, multi-user CoMP (MU-CoMP) should be implemented to support CoMP transmission to a plurality of UEs in the same resources. However, if each of UEs for which a CoMP operation is performed should decode traffic destined for the other UEs as well as traffic destined for the UE in MU-CoMP, many problems may occur in terms of security, hardware complexity, power consumption, etc.